Third Eyelid Showing in Cats - Causes and Treatment

Third Eyelid Showing in Cats - Causes and Treatment

Cats' eyes can be truly fascinating for those who take their time to look into them. Not only due to the fabulous combinations of colors, but also due to the unusual behavior of their pupils, which change in size depending on the amount of light in the environment. Their eyes are so well-known for their distinctive appearance, reflection spots on roads are known as cat's eyes.

A cat's eye contains an inner membrane known as the third eyelid. This is to differentiate it from the outer eyelids. You may have never seen it, as doing so is a sign of a health problem. As such, if you've recently noticed it, you will need to read up on the causes and treatment for a third eyelid showing in cats. Fortunately, AnimalWised provides the information to do so. It is a symptom of a heath problem in your cat and should not be ignored under any circumstances.

What is the third eyelid?

The scientific name of this membrane located in the eyes of most mammals, including cats, is the nictitating membrane (palpebra tertia). More commonly it is known as the third eyelid. It is a tissue located around the cornea, conjunctiva and mucous membrane.

Although you may not notice, you can often see secretions between the eye and the outer eyelids when your cat is half asleep. As they wake up, a cat's third eyelid showing as they begin to open their eyes is normal. However, it should not be visible once open, awake and alert.

One role of the third eyelid is to protect the eyeballs from any foreign object and from traumas as a result of a hit or knock. Their action is instinctive and automatic. It is also responsible for releasing a liquid with antiseptic properties to fight bacteria and microorganisms that could cause disease. However, these protective uses of the third eyelid should not lead to the feline nictitating membrane to show all the time.

Nictitating membranes are also sometimes known as a 'haw'. In other animals, including many birds, it can be used to moisten the eyeball while open. As it is transparent or translucent, it means the animal can still see. Some mammals have a full third eyelid which can do this, but the cat does not. It is believed that some species have evolved away from using this membrane. Even humans have a vestigial part of their conjunctiva known as the plica semilunaris which is thought to have once been a third eyelid.

If you see that your cat's third eyelid is visible, either in one eyeball or in both, this means that they are in pain or suffering from a condition or disease[1]. Some research suggests that the appearance of this membrane is related to intestinal parasites and digestive problems, whereas others state that if this membrane is visible then the problem is related to the eye itself. In these latter cases, the problem with the cat's inner eyelid could be due to a prolapse[2]. In order for you to have all the information, we will show you the causes that can cause this tissue to appear in your cat's eye.

Causes of third eyelid showing in cats

If you see that your cat's nictitating membrane is visible in one or both eyes, it is usually a symptom of a secondary problem. These are some of the root causes of third eyelid showing in cats:

  • Conjunctivitis: this ocular infection doesn't only cause the appearance of the third eyelid, but is also causes swelling, tearing and lacrimation (tearing).
  • Dehydration: when the cat is suffering from a severe lack of water, the appearance of this ocular tissue is a sign and symptom of a serious health condition. They will need to be treated by a specialist who will need to replace fluids and treat the underlying cause of dehydration if necessary.
  • Wounds: if your cat has suffered a blow or trauma to the face, both of its eyes may have become affected. Infection is also possible. Take them to the vet straight away to treat infection and repair any damage.
  • Foreign body: anything that gets into the cat's eyes, whether rubbish, dust or anything else, will cause the nictitating membrane to appear, as it is a mechanism to prevent it from further embedding into the eyeball.
  • Cancer: it's possible for this small organ to be affected by the formation of cancer cells. Fortunately, the disease is relatively rare in this part of the eye. Unfortunately, if it does occur, then it can progress quickly and become fatal[3].
  • Haw's syndrome: this name is given to the appearance of the third eyelid when related to an intestinal problem from which the cat is suffering or has suffered from recently. Examples include intense diarrhea or the presence of parasites. With Haw's syndrome in cats, the third eyelid turns into a protrusion thanks to prolapse, most commonly occurring in cats less than 2 years of age. Prolapse of the nictitating membrane is one of the most common issues with third eyelids in dogs[4].
  • Parasites: an infestation can be one of the causes of haw's syndrome, but the parasites itself are the problem. Eyeworms can lodge themselves in the eye, potentially resulting in blindness if not treated promptly.
  • Genetics: some cat breeds, such as the Burmese, might be prone to suffering a rupture or protrusion of this membrane inside the eye, which is very uncomfortable for them.
  • Dermoid cyst: a dermoid cyst is one which is present at birth and can grow slowly as they age. It is usually not observed until secondary symptoms occur. One case study on a kitten with third eye showing from 2016 shows that it can be present on any part of the eye. The result is still protrusion of the nictitating membrane[5]. However, the same study notes that any ocular dermoid occurs infrequently and can respond well to treatment.

The length of time the third eyelid is showing will correlate to the cause. As we said, it may appear for moments after sleeping. However, even once the cat begins treatment, the nictitating membrane may still be visible. This is because the cat's body is taking time to heal.

Furthermore, if you see the third eyelid in your cat it is also highly likely that its eyes will start to tear. They will probably try to touch them with their paws owing to the discomfort caused when this membrane is out of place. So if we see our cat scratching at their face or rubbing it too frequently, we should look to see if we can observe the third eyelid showing.

Third eyelid showing in one eye

As a note, if a cat's inner eyelid is showing in one eye and not the other, it is still a cause for concern. It may imply a problem on one side of the body, but it still means there is an issue which needs resolving. It might be that you see a cat's third eyelid showing in one eye which progresses to both. This implies the progression of one of the causes listed above.

If a cat third eyelid showing in one eye is observed, it could be some direct damage to this membrane. This would explain why it is visible in one eye and not the other. It could be a piece of dust which has lodged within or even something hitting it. As we have stated above, cancer is a possible cause of protrusion in the third eyelid, but this is relatively rare. Regardless if a cat's nictitating membrane is observed in one eye or two, you should take them to the vet for evaluation.

Treatment for a third eyelid showing in cats

Owing to the various reasons that can cause the cat's nictitating membrane to occupy more space in the eye than it should, there are several types of treatment. This treatment depends on the source of the problem.

In the case of dehydration causing your cat's third eyelid showing, you should give the cat plenty of wet food and water in order to halt the process. It is also recommended to take the cat to the vet, as the animal has been lacking water for too long to be treated at home.

In the case of conjunctivitis, wounds, foreign bodies in the eye and cancer, only the vet's diagnosis will be able to determine the next step to follow. They might be prescribed eye drops and other medications in the first 3 problems, depending on the severity. If they have cancer they may be recommended surgery and/or radiotherapy. Surgical removal may occur if there is a benign tumor on the cat's inner eyelid. Only the specialist can decide on the best option to conserve the cat's health and quality of life. As the cat recovers, their inner eyelid should start to retract.

Haw's syndrome should disappear by itself, once the intestinal and digestive problems that caused the appearance of the membrane go away. This, in turn, will lead to the nictitating membrane gradually disappearing as the cat returns to health.

When a genetic reason is behind it, the vet will use medical studies to determine whether the cat's inner eyelid membrane is affecting the cat's vision and causing discomfort. If this is the case the vet may also resort to surgery. This process will not to extract their third eyelid, but will put the feline nictitating membrane back into its correct position. If there are any other infections or conditions causing a cat's third eyelid to show, they will be treated accordingly.

The protrusion of the third eye in cats is not the only way the cat's eye can tell us about their health and well-being. Our video below helps explain why dilated pupils in a cat's eye are also revelatory:

If you want to read similar articles to Third Eyelid Showing in Cats - Causes and Treatment, we recommend you visit our Eye care category.


1. Muir, P., et al. (1990). A Clinical and Microbiological Study of Cats with Protruding Nictitating Membranes and Diarrhoa. Vet Rec, 127(13), 324-330.

2. Williams, D., & Middleton, S. (2011). Everted Third Eyelid Cartilege in a Cat: A Case Report and Literature Review. Veterinary Ophthalmology, 15(2), 123-127.

3. Komáromy, A. M., et al. (1997). Primary Adenocarcinoma of the Gland of the Nictitating Membrane in a Cat. Journal of the American Animal Hospital Association, 33, 333-336.

4. Edlemann, M. L., et al. (2013). Investigating the Inheritance of Prolapsed Nictitating Membrane Glands in a Large Canine Pedigree, Veterinary Ophthalmology, 16(6), 416-422.

5. LoPinto, J. A. (2016). Dorsoally Located Corneal Dermoid in a Cat. JFMS Open Rep, 2(1).